Incident light is reflected on the surfaces of a spectacle lens. The proportion of the reflected light is dependent on the incident angle and the refractive index of the lens. Thus, for example, the proportion of light reflected on each surface of an untreated spectacle lens having a refractive index of 1.5 with perpendicular incident light is about 4%, so that only about 92% of the incoming light is transmitted through the spectacle lens. The light reflected by the spectacle lens causes disturbing glare on its front side as well as its back side facing the wearer of the spectacles. The reflections on the front side of the spectacle lens are perceived as disturbing by a person standing opposite the wearer of the spectacles. The light reflected by the back side causes the person wearing the spectacles to perceive, among other unwanted optical effects, a blurred image.
Antireflections for the most part eliminate these disturbing reflections because they increase the amount of light passing through the spectacle lens. Spectacle lenses with high-quality antireflection on the front and back sides have a light transmittance of about 98%, which corresponds approximately to a light reflectance of 1% per surface.
Usually, the antireflection is achieved with the assistance of a so-called antireflection coating. The reduction of reflections hereby is based on the principle of interference. The antireflection coating comprises a plurality of thin layers which are several ten nanometers thick and are monolithically arranged on top of each other with different refractive indexes. The light waves reflected by the coating surface are superimposed and ideally cancel each other out.
Because of the finite number of layers in an antireflection coating, it is not possible to completely eliminate the reflection over the entire visible spectrum.
Therefore, antireflection coatings generally lead to a colored residual reflection. The reflection color is determined by the location of the maximum of the reflection curve as a function of the wavelength.
One quality criterion of antireflection coatings for spectacle lenses is the color of the residual reflection. This determines the cosmetic appearance of the spectacle lens to quite some extent. The usual antireflection coating of spectacle lenses has a green residual reflection. For example, the reflection of a surface of the antireflective spectacle lens has a value between 0.75% and 1.5% at a wavelength of 500 nm. United States patent application publication 2007/0202251 A1, on which the invention is based, discloses a spectacle lens with an antireflection coating which, under illumination of a natural daylight spectrum, causes a color-neutrally appearing residual reflection.
The antireflection coating disclosed in United States patent application publication 2007/0202251 A1 has proven itself for the application described therein. The application disclosed therein relates to an antireflection coating which is applied directly to the substrate, for example, a spectacle lens.
Modern coatings for plastic spectacle lenses are configured differently than described in United States patent application publication 2007/0202251 A1. The antireflection coating is not applied directly to the spectacle lens. Instead, it is provided with a scratch-resistant layer beforehand. This is necessary since plastics for spectacle lenses such as polythiourethane have a very low scratch resistance. To make the spectacle lens product scratch-resistant, spectacle lenses are, for example, coated with a hard lacquer in an immersion process. After hardening of the hard lacquer the coated spectacle lenses have a high scratch resistance. The thickness of such a hardened layer is typically between 2 and 4 micrometers.
In many spectacle lens products, the antireflection coating is applied on the scratch-resistant layer.
Even though the antireflection coating described in United States patent application publication 2007/0202251 A1 has essentially proven effective also for plastic spectacle lenses with scratch-resistant coating and antireflection coating, it has been found that while the residual reflection is not consciously perceived in natural sunlight it is perceived to be disturbing, for example, for both the wearer of the spectacles and the person standing opposite the wearer of the spectacles in rooms with fluorescent tube lighting.